Introduction to Digital Signature

You may have signed your name on an application for admission, legal documents, or credit card receipts, indicating that you agree with the content of these documents. In the field of data security, the device needs to identify the sender or the producer of the data to indicate that the data is not forged, has been authorised, is legitimate, and can be used safely. Digital signature is a technical solution for verifying data legitimacy.

Digital signature has two properties: unforgeability, that is, only legitimate data senders can sign data, and other signatures are invalid; verifiability, that is, data users must be able to verify the validity of the signature.

Common digital signature algorithms include RSA and DSA. The basic process of digital signature verification is:

  1. The data sender generates a private key, which is used to generate a public key, thus getting a private-public key pair. Note that only the private key can generate a matching public key.

  2. The data sender saves the public key to the storage system of the data user.

  3. The data sender signs the data with the private key and sends the signed data and signature to the data user.

  4. After receiving the data, the data user uses the public key stored in step 2 to verify the signature sent in step 3. If the signature is correct, the data is considered to be from a legitimate data sender; otherwise, the data is considered unauthorised and will not be used.

The basic principle of using digital signature to verify data legitimacy is shown in Figure 13.10.

Figure 13.10. Basic principle of using digital signatures to verify data legitimacy

Through this mechanism of "private key issuing public key, private key signing, and public key verifying signature", the legitimacy of the data source can be authenticated. However, you may have noticed that there are prerequisites for this scheme to be effective:

  • The private key of the data sender should not be leaked. Once the private key is made public, an attacker can use the public private key to sign illegal data and send it to the data user, then this verification mechanism will fail.

  • The public key of the data user cannot be removed at will. If an attacker generates a private-public key pair on his own system and replaces the public key of the data user with his own public key, the attacker can sign illegal data with his own private key and then send the illegal data to the data user. The data user may use the replaced public key to verify the data sent by the attacker, and thus consider the data to be legitimate.

In the following sections, we will see how the Secure Boot scheme is designed to address these issues. Let's continue our journey to the next section!